The present invention relates to a scanning electron microscope for detecting a signal generated from a sample by irradiation of an electron beam and forming a scanned image of the sample.
A scanning electron microscope is an apparatus for scanning an electron beam which is generated from an electron source and finely limited by a focusing lens and an object lens on a sample using a deflector, detecting a signal generated from the sample by irradiation of the electron beam by a detector, and forming a sample image by processing the detection signal in synchronization with scanning of the electron beam. In order to improve the yielding rate of semiconductor devices, it is important to observe and analyze foreign substances and faults on devices. In correspondence with recent super refinement of semiconductor elements, observation and analysis of fine foreign substances and faults on semiconductor devices are required and for observation of foreign substances and faults, a scanning electron microscope is used instead of an optical microscope.
Information on irregularity such that observed foreign substances and faults are hollowed or projected is important information for analysis of foreign substances and faults. As a method for obtaining irregularity information of a sample, as described in Japanese Patent Publication 6-043885, there is a method for detecting a signal emitted in a direction at a small angle with the sample surface and forming a sample image. To observe fine foreign substances and faults, improvement of the resolution of a scanning electron microscope is desired. As a means for improvement of the resolution, by a method for bringing a sample close to an object lens or for leaking the magnetic field of an object lens on the sample side, the distance between the main surface of the object lens and the sample is made shorter.
According to the aforementioned prior art, a detector for detecting a signal emitted in a direction at a small angle with the sample surface must be arranged so as to look steadily at the electron beam irradiation position onto the sample and is inevitably arranged between the object lens and the sample. However, when the distance between the object lens and the sample is made shorter for improvement of the resolution, a problem arises that the amount of signals which can be detected by the detector is reduced and the SN ratio of sample images lowers. When an object lens of a magnetic field leakage type is used for improvement of the resolution, a problem arises that the track of a signal generated from a sample is bent by the magnetic field of the object lens and the signal cannot arrive at the detector.
The present invention was developed in consideration of the aforementioned problems of the prior arts and is intended to provide a scanning electron microscope for observing images at high resolution and obtaining irregularity information of a sample.
When an electron beam is irradiated onto the surface of a sample which is uneven due to foreign substances, the emission direction of reflected electrons emitted in a direction at a small angle with the sample surface from the uneven inclined parts is directive. Namely, most of the reflected electrons are emitted in the direction of the inclined surface and few reflected electrons are emitted in the backward direction of the inclined surface. Therefore, in a reflected electron image formed by detecting reflected electrons emitted in a specific direction from the sample, information concerning the inclination direction of inclined surface existing on the sample surface is included. By analysis of this inclination direction, irregularity information of the sample surface is obtained.
To detect reflected electrons emitted in a direction at a small angle with the sample surface by a scanning electron microscope having a short distance between the main surface of an object lens and a sample, according to the present invention, a detector is arranged on the side of an electron source of a magnetic field leakage type object lens and by controlling any one of or both of a negative voltage applied to the sample and an acceleration voltage for accelerating reflected electrons emitted from the sample, reflected electrons emitted in a direction at a small angle with the sample surface are detected. Reflected electrons emitted in a direction at a small angle with the sample surface are emitted from the sample by applying a negative voltage to the sample, and the track thereof is bent in the direction of the optical axis of the electron beam by the leakage magnetic field of the object lens, and the reflected electrons pass through the object lens by the acceleration electric field and is detected by the detector arranged on the electron source side by the object lens.
Namely, the scanning electron microscope of the present invention is characterized in that it has an electron source for generating an electron beam, a focusing lens for focusing the electron beam, a magnetic field type object lens for finely limiting the focused electron beam and irradiating it onto a sample, a deflector for two-dimensionally scanning the electron beam on the sample, a detector arranged on the electron source side of the object lens so as to detect a signal emitted from the sample by irradiation of the electron beam, a display means for displaying the signal detected by the detector as a sample image, a deceleration electric field generation means for generating an electric field for decelerating the electron beam to be irradiated onto the sample, and a voltage control means for controlling a voltage applied to the deceleration electric field generation means, and the voltage control means controls the voltage to be applied to the deceleration electric field generation means so that reflected electrons emitted in a direction at a small angle with the sample surface among the signal generated from the sample are detected by the detector, and a sample image having effects of light and shade in correspondence with the irregularity of the sample surface can be obtained.
The deceleration electric field generation means generates a deceleration electric field by applying a negative voltage to a sample. The deceleration electric field generation means may include an acceleration electric field generation means for generating an electric field for accelerating reflected electrons generated from the sample in the direction of the electron source and a voltage control means for controlling a voltage to be applied to the acceleration electric field generation means.
The scanning electron microscope of the present invention is also characterized in that it has an electron source for generating an electron beam, a focusing lens for focusing the electron beam, a magnetic field type object lens for finely limiting the focused electron beam and irradiating it onto a sample, a deflector for secondarily scanning the electron beam on the sample, a detector arranged on the electron source side of the object lens so as to detect a signal emitted from the sample by irradiation of the electron beam, a display means for displaying the signal detected by the detector as a sample image, an acceleration electric field generation means for generating an electric field for accelerating the signal generated from the sample, and a voltage control means for controlling a voltage to be applied to the acceleration electric field generation means, and the voltage control means controls the voltage to be applied to the acceleration electric field generation means so that reflected electrons emitted in a direction at a small angle with the sample surface among the signal generated from the sample are detected by the detector, and a sample image having effects of light and shade in correspondence with the irregularity of the sample surface can be obtained.
The object lens of the scanning electron microscope of the present invention may be of a type of generating a leakage magnetic field in a sample atmosphere.
It is possible to install a plurality of detectors for detecting reflected electrons emitted in a direction at a small angle with the sample surface in symmetrical positions about the optical axis of the electron beam and obtain sample images having different effects of light and shade for irregularity of the sample surface by detection signals of the detectors. Although irregularity information of the sample can be obtained only by one detector, when a pair of detectors are installed in symmetrical positions about the optical axis of the electron beam and two sample images on the basis of detection signals of the detectors are compared, the irregularity of the sample can be evaluated more precisely. The shade of a reflected electron image also appears due to a local change of the material of the sample. The shade of a reflected electron image caused by a local change of the material appears in the same position of each of the aforementioned two sample images in the same way. On the other hand, the brightness and darkness of a reflected electron image caused by the shape of the sample, that is, irregularity are reversed in the aforementioned two reflected electron images such that the bright position of one reflected electron image is the dark position of another reflected electron image. Therefore, when the positions in the two reflected electron images where the brightness and darkness are reversed are identified and the light and shade positions and the emission direction of reflected electrons contributing to forming of the images are combined and analyzed, the direction of the inclined surface existing on the sample surface can be known and furthermore, whether the position held between the light portion and the dark portion is projected from the sample surface or hollowed in the sample surface can be known.
The brightness and darkness of a sample image, in consideration of rotation of reflected electrons detected by the detector due to the magnetic field of the object lens, must correspond to the irregularity of the sample surface. In order to analyze the irregularity of the sample surface on the basis of the brightness and darkness of the sample image, it is necessary to confirm the emission direction of reflected electrons to the sample which are detected by the detector instead of the location relationship of the detector in the scanning electron microscope and analyze the brightness and darkness of the sample image according to the relationship between the sample and the reflected electrons emission direction. The rotary rate of reflected electrons in the magnetic field of the object lens can be known from the exciting current of the object lens and the acceleration voltage of the primary electron beam and on the basis of the rotary rate, the direction in which reflected electrons are emitted from the sample which are detected by the detector can be known.
Instead of directly detecting reflected electrons emitted in a direction at a small angle with the sample surface by the detector, it is possible to allow reflected electrons to collide with a conductor plate once and detect secondary electrons generated from the conductor plate. Namely, a conductor plate having an opening for passing the electron beam is installed on the electron source side of the object lens and the opening size of the conductor plate and the intensity of the deceleration electric field or the acceleration electric field may be set so that most of secondary electrons emitted from a sample pass through the opening and reflected electrons emitted in a direction at a small angle with the sample surface collide with the conductor plate. In this case, in a position which is expected as a detection direction of secondary electrons emitted from the conductor plate due to collision of reflected electrons, the detector is arranged.
It is also possible to install a second detector on the electron source side of the aforementioned detector for detecting reflected electrons emitted in a direction at a small angle with the sample surface and detect any one of or both of secondary electrons generated from the sample and reflected electrons emitted in a direction at a large angle with the sample surface by the second detector. By using a detection signal of the second detector, a general scanning electron microscope image can be displayed on the display means.
On the display means, a sample image by reflected electrons emitted in a direction at a small angle with the sample surface can be displayed. Irregularity information of the sample surface can be displayed on the display means. Irregularity information of the sample surface may be displayed independently of a sample image and may be overlaid on a sample image.